Nonlinear polarization amplifiers in nonzero dispersion shifted fiber

ABSTRACT

A broadband fiber transmission system includes a transmission line with at least one zero dispersion wavelength λ o  and transmits an optical signal of λ. The transmission line includes a distributed Raman amplifier that amplifies the optical signal through Raman gain. One or more semiconductor lasers are included and operated at wavelengths λ p  for generating a pump light to pump the Raman amplifier. λ is close to λ o  and λ 0  is less than 1540 nm or greater than 1560 nm.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of 09/760,201, filed Jan. 12,2001, and 09/565,776, filed May 5, 2000, now U.S. Pat No. 6,239,902which is a continuation of 09/046,900 filed Mar. 24, 1998 now U.S. Pat.No. 6,101,024. All the above applications are fully incorporated hereinby reference.

BACKGROUND

1. Field of the Invention

The present invention relates generally to nonlinear polarizationamplifiers, and more particularly to nonlinear polarization amplifiersused to amplify signals propagating in non-zero dispersion shiftedoptical fibers.

2. Description of the Related Art

Because of the increase in data intensive applications, the demand forbandwidth in communications has been growing tremendously. In response,the installed capacity of telecommunication systems has been increasingby an order of magnitude every three to four years since the mid 1970s.Much of this capacity increase has been supplied by optical fibers thatprovide a four-order-of-magnitude bandwidth enhancement overtwisted-pair copper wires.

To exploit the bandwidth of optical fibers, two key technologies havebeen developed and used in the telecommunication industry: opticalamplifiers and wavelength-division multiplexing (WDM). Opticalamplifiers boost the signal strength and compensate for inherent fiberloss and other splitting and insertion losses. WDM enables differentwavelengths of light to carry different signals parallel over the sameoptical fiber. Although WDM is critical in that it allows utilization ofa major fraction of the fiber bandwidth, it would not be cost-effectivewithout optical amplifiers. In particular, a broadband optical amplifierthat permits simultaneous amplification of many WDM channels is a keyenabler for utilizing the full fiber bandwidth.

Silica-based optical fiber has its lowest loss window around 1550 nmwith approximately 25 THz of bandwidth between 1430 and 1620 nm. Forexample, FIG. 1 illustrates the loss profile of a 50 km optical fiber.In this wavelength region, erbium-doped fiber amplifiers (EDFAs) arewidely used. However, as indicated in FIG. 2, the absorption band of aEDFA nearly overlaps its the emission band. For wavelengths shorter thanabout 1525 nm, erbium-atoms in typical glasses will absorb more thanamplify. To broaden the gain spectra of EDFAs, various dopings have beenadded. For example, as shown in FIG. 3a, codoping of the silica corewith aluminum or phosphorus broadens the emission spectrum considerably.Nevertheless, as depicted in FIG. 3b, the absorption peak for thevarious glasses is still around 1530 nm.

Hence, broadening the bandwidth of EDFAs to accommodate a larger numberof WDM channels has become a subject of intense research. As an exampleof the state-of-the-art, a two-band architecture for an ultra-widebandEDFA with a record optical bandwidth of 80 nm has been demonstrated. Toobtain a low noise figure and high output power, the two bands share acommon first gain section and have distinct second gain sections. The 80nm bandwidth comes from one amplifier (so-called conventional band orC-band) from 1525.6 to 1562.5 nm and another amplifier (so-called longband or L-band) from 1569.4 to 1612.8 nm. As other examples, a 54 nmgain bandwidth achieved with two EDFAs in a parallel configuration,i.e., one optimized for 1530-1560 nm and the other optimized for1576-1600 nm, and a 52 nm EDFA that used two-stage EDFAs with anintermediate equalizer have been demonstrated.

These recent developments illustrate several points in the search forbroader bandwidth amplifiers for the low-loss window in optical fibers.First, bandwidth in excess of 40-50 nm require the use of parallelcombination of amplifiers even with EDFAs. Second, the 80 nm bandwidthmay be very close to the theoretical maximum. The short wavelength sideat about 1525 nm is limited by the inherent absorption in erbium, andlong wavelength side is limited by bend-induced losses in standardfibers at above 1620 nm. Therefore, even with these recent advances,half of the bandwidth of the low-loss window, i.e., 1430-1530 nm,remains without an optical amplifier.

There is a need for nonlinear polarization amplifiers that provide a lownoise figure amplification for operation near the zero dispersionwavelength of fibers. There is a further need for a broadband fibertransmission system that includes nonlinear polarization amplifierswhich provide low noise amplification near the zero dispersionwavelength of fibers.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide nonlinearpolarization amplifiers.

Another object of the present invention is to provide a broadband fibertransmission system with at least one nonlinear polarization amplifier.

Yet another object of the present invention is to provide a broadbandfiber transmission system with reduced fiber non-linear impairments.

A further another object of the present invention is to provide abroadband fiber transmission system that operates over the full low losswindow of available and optical fibers.

Another object of the present invention is to provide a broadband fibertransmission system that uses distributed Raman amplification to lowersignal power requirements.

These and other objects of the present invention are achieved in abroadband fiber transmission system. The broadband fiber transmissionsystem includes a transmission line with at least one zero dispersionwavelength λ_(o) and transmits an optical signal of λ. The transmissionline includes a distributed Raman amplifier that amplifies the opticalsignal through Raman gain. One or more semiconductor lasers are includedand operated at wavelengths λ_(p) for generating a pump light to pumpthe Raman amplifier. λ is close to λ₀ and λ₀ is less than 1540 nm orgreater than 1560 nm.

In another embodiment of the present invention, a broadband fibertransmission system is provided. A transmission line includes at leastone zero dispersion wavelength λ_(o) and transmits an optical signal ofλ. The transmission line includes a distributed Raman amplifier and adiscrete optical amplifier that amplify the optical signal of λ. One ormore semiconductor lasers are included and operated at wavelengths λ_(p)for generating a pump light to pump the amplifiers. λ is close to λ₀ andλ₀ is less than 1540 nm or greater than 1560 nm.

In another embodiment of the present invention, a method of broadbandamplification provides a broadband fiber transmission system with atransmission line having at least one zero dispersion wavelength λ_(o).The transmission line includes a distributed Raman amplifier thatamplifies an optical signal through Raman gain. An optical signal of λis transmitted. The Raman amplifier is pumped with pump light λ_(p). λis close to λ₀ and λ₀ is less than 1540 nm or greater than 1560 nm.

In another embodiment of the present invention, a method of broadbandamplification provides a broadband fiber transmission system with atransmission line having at least one zero dispersion wavelength λ_(o).The transmission line includes a distributed Raman amplifier and adiscrete optical amplifier that amplify an optical signal of λ. Anoptical signal of λ is transmitted. The Raman amplifier and discreteoptical amplifiers are pumped with pump light λ_(p). λ is close to λ₀and λ₀ is less than 1540 nm or greater than 1560 nm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot of loss verses wavelength for 50 km fiber and the gainband of a typical EDFA.

FIG. 2 is a graphical illustration of absorption and gain spectra of anEDFA.

FIG. 3a is a graphical illustration of emission spectra of four EDFAswith different core compositions.

FIG. 3b is a graphical illustration of absorption cross-section oferbium-doped glass of different compositions.

FIG. 4 is a graphical illustration of a measured Raman-gain spectrum forfused silica at a pump wavelength of 1000 nm.

FIG. 5 is a graphical illustration that plots power gain coefficient 2 gversus phase vector mismatch Δk for parametric amplification.

FIG. 6 is a schematic diagram of one embodiment of a nonlinearpolarization amplifier of the present invention.

FIG. 7 is a graphical illustration of spectral broadening and gainexpected from parametric amplification for a pump power of 1 W anddifferent separations between the pump and zero-dispersion wavelength.

FIG. 8 is a graphical illustration of spectral broadening and gainexpected from parametric amplification for a pump and zero-dispersionwavelength separation of 1 nm and for varying pump powers.

FIG. 9 is a schematic diagram of an embodiment of a broadband fibertransmission system of the present invention using an open-loopconfiguration.

FIG. 10 is a schematic illustration of a broadband fiber transmissionsystem of the present invention using a Sagnac Raman cavity that ispumped at 1240 nm.

FIG. 11 is a schematic illustration of an embodiment of a broadbandfiber transmission system of the present invention using a Sagnac Ramancavity that is pumped at 1117 nm.

FIG. 12 is a schematic illustration of an embodiment of a broadbandfiber transmission system of the present invention with two stages ofnonlinear polarization amplifiers.

FIG. 13 is a schematic illustration of an embodiment of a broadbandfiber transmission system of the present invention that is a combinationof an EDFA and a nonlinear polarization amplifier.

FIG. 14 is a schematic diagram of one embodiment of a dual stageamplifier.

FIG. 15 is a graph of gain versus wavelength for an S band dual stageamplifier, such as for the embodiment of FIG. 14.

FIG. 16 is a graph of noise figure versus wavelength for an S band dualstage amplifier, such as for the embodiment of FIG. 14.

FIG. 17 is a block chart of various embodiments of uses of amplifiers.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Some embodiments provide a structure for exploiting almost the full 25THz of bandwidth available in the low-loss window of optical fibers from1430 nm to 1620 nm. The broadband NLPA amplifier of some embodimentscombines Raman amplification with either PA or 4WM to achieve bandwidthperformance improvements that neither technology by itself hasheretofore been able to deliver.

The broadband NLPA of other embodiments comprise an input port forinputting an optical signal having a wavelength λ, a distributed gainmedium for receiving the optical signal and amplifying and spectrallybroadening the same therein through nonlinear polarization, a pumpsource operated at wavelength λ_(p) for generating a pumping light topump the distributed gain medium, and an output port for outputting theamplified and spectrally broadened optical signal. The distributed gainmedium can have zero-dispersion at wavelength λ₀ such that λ≧λ₀≧λ_(p).The pumping light can cascade through the distributed gain medium aplurality of Raman orders including an intermediate order having awavelength λ_(r) at a close proximity to the zero-dispersion wavelengthλ₀ to phase match four-wave mixing (if λ_(r)<λ₀) or parametricamplification (if λ_(r)>λ₀).

A first embodiment of the NLPA uses open-loop amplification with anoptical fiber gain medium. A pump source operated at 1240 nm can beused. The pump may be retro-reflected to increase the conversionefficiency. A second embodiment of the NLPA can use a Sagnac Ramancavity that is pumped at 1240 nm. Feedback in the Sagnac Raman cavitycan reduce the required pump power, and the broadband cavity designsupports much of the generated bandwidth. Another embodiment of the NLPAcan use a Sagnac Raman cavity pumped at 1117 nm for a very broadbandoperation.

Other embodiments relate to a parallel optical amplification apparatushaving a combination of optical amplifiers. In one embodiment, theparallel optical amplification apparatus comprises two parallel stagesof NLPAs with one NLPA optimized for 1430 to 1480 nm and the other for1480 to 1530 nm. In another embodiment, the full 25 THz of the low-losswindow in optical fibers can be exploited by a parallel combination of aRaman amplifier and a rare earth doped amplifier. In one embodiment, anNLPA can cover the low-loss window of approximately 1430 nm to 1530 nm,and an EDFA can cover the low-loss window of approximately 1530 nm to1620 nm.

Stimulated Raman scattering effect, PA and 4WM can be result ofthird-order nonlinearities that occur when a dielectric material such asan optical fiber is exposed to intense light. The third-order nonlineareffect can be proportional to the instantaneous light intensity.

Stimulated Raman scattering can be an important nonlinear process thatturns optical fibers into amplifiers and tunable lasers. Raman gain canresult from the interaction of intense light with optical phonons insilica fibers, and Raman effect leads to a transfer of energy from oneoptical beam (the pump) to another optical beam (the signal). The signalcan be downshifted in frequency (or upshifted in wavelength) by anamount determined by vibrational modes of silica fibers. The Raman gaincoefficient g, for the silica fibers is shown in FIG. 4. Notably, theRaman gain g, can extend over a large frequency range (up to 40 THz)with a broad peak centered at 13.2 THz (corresponding to a wavelength of440 cm⁻¹). This behavior over the large frequency range can be due tothe amorphous nature of the silica glass and enable the Raman effect tobe used in broadband amplifiers. The Raman gain can depend on thecomposition of the fiber core and can vary with different dopantconcentrations.

Raman amplification has some attractive features. First, Raman gain canupgrade existing fiber optic links because it is based on theinteraction of pump light with optical phonons in the existing fibers.Second, in some embodiments there is no excessive loss in the absence ofpump power—an important consideration for system reliability.

Raman cascading is the mechanism by which optical energy at the pumpwavelength is transferred, through a series of nonlinear polarizations,to an optical signal at a longer wavelength. Each nonlinear polarizationof the dielectric can produce a molecular vibrational statecorresponding to a wavelength that is offset from the wavelength of thelight that produced the stimulation. The nonlinear polarization effectcan be distributed throughout the dielectric, resulting in a cascadingseries of wavelength shifts as energy at one wavelength excites avibrational mode that produces light at a longer wavelength. Thisprocess can cascade through numerous orders. Because the Raman gainprofile can have a peak centered at 13.2 THz in silica fibers, one Ramanorder can be arranged to be separated from the previous order by 13.2THz.

Cascading makes stimulated Raman scattering amplifiers very desirable.Raman amplification can be used to amplify multiple wavelengths (as inwavelength division multiplexing) or short optical pulses because thegain spectrum can be very broad (a bandwidth of greater than 5 THzaround the peak at 13.2 THz). Cascading can enable Raman amplificationover a wide range of different wavelengths. By varying the pumpwavelength or by using cascaded orders of Raman gain, the gain can beprovided over the entire telecommunications window between 1300 nm and1600 nm.

Parametric amplification and 4 wave mixing (PA/4 WM) involve two pump(P) photons that create Stokes (S) and anti-Stokes (A) photons. BothPA/4WM and Raman amplification arise from the third order susceptibilityχ⁽³⁾ in optical fibers. More specifically, the real part of χ⁽³⁾, theso-called nonlinear index of refraction n₂, is responsible for PA/4WM,while the imaginary part of χ⁽³⁾ associated with molecular vibrationscorresponds to the Raman gain effect. In silica fibers of someembodiments, about ⅘ths of the n₂ is an electronic, instantaneousnonlinearity caused by ultraviolet resonances, while about ⅕th of n₂arises from Raman-active vibrations, e.g., optical phonons. Theimaginary part of this latter contribution corresponds to the Raman gainspectrum of FIG. 4.

Whereas Raman amplification is attractive for providing optical gain,PA/4WM can offer an efficient method to broaden the bandwidth of theoptical gain. PA/4WM can have a much smaller frequency separationbetween pump and signal than Raman amplification, and the frequencydifference may depend on the pump intensity. As in Raman amplification,one advantage of PA/4WM gain is that it can be present in every fiber.However, unlike the Raman effect, both PA and 4WM can requirephase-matching. 4WM can be inefficient in long fibers due to therequirement for phase-matching. However, PA can act asself-phase-matched because the nonlinear index of refraction is used tophase match the pump and sidebands. This can be true in embodimentsoperating near the zero-dispersion wavelength in fibers. When 4WM and PAoccur near the zero-dispersion wavelength of a single-mode fiber,phase-matching can become automatic in the fiber. In 4WM, sidebands canbe generated without gain when the pump wavelength falls in the normaldispersion regime (where the pumping wavelength is shorter than thezero-dispersion wavelength). PA is 4-photon amplification in which thenonlinear index of refraction is used to phase match the pump andsidebands. For PA the pump wavelength can lie in the anomalous groupvelocity regime (i.e., where the pumping wavelength is longer than thezero-dispersion wavelength) and proper phase matching can require thatpump and signal be co-propagating in some embodiments.

To illustrate the PA/4WM gain, the gain coefficient can be derived as:$\begin{matrix}{g = \sqrt{\left( {\gamma \quad P} \right)^{2} - \left\lbrack {\left( \frac{\Delta \quad \kappa}{2} \right) + {\gamma \quad P}} \right\rbrack^{2}}} & 1\end{matrix}$

The first term under the square root sign corresponds to the third ordernonlinearity that couples the pump photons to the sidebands. The secondterm corresponds to the phase mismatch between the waves and it consistsof two parts: one due to the wave-vector mismatch at the differentwavelengths and the other due to the increase in nonlinear index inducedby the pump. The nonlinearity parameter can be defined as$\begin{matrix}{\gamma = {{\frac{\omega}{c}\frac{n_{2}}{A_{eff}}} = {\frac{2\pi}{\lambda}\frac{n_{2}}{A_{eff}}}}} & 2\end{matrix}$

Some embodiments operate near the zero-dispersion wavelength λ₀, and thepropagation constant can be expanded as: $\begin{matrix}{{\Delta \quad \kappa} = {{- {\frac{\lambda^{2}}{2\pi \quad c}\left\lbrack {{{\frac{D}{\lambda}}}_{\lambda_{0}}\left( {\lambda_{p} - \lambda_{0}} \right)} \right\rbrack}}\Omega^{2}}} & 3\end{matrix}$

where

Ω=_(ω) _(p) −_(ω) _(s) =_(ω) _(d) −_(ω) _(p) .  4

The pump wavelength can falls in the normal dispersion regime for someembodiments, and D<0, ∂D/∂λ>0, (λ_(p)−λ₀)<0, so that Δk>0. In this case,g can be imaginary, and there may be no gain during the sidebandgeneration process. This can correspond to the case of 4WM. Someembodiments operate in the anomalous group velocity dispersion regime,and D>0, ∂D/∂λ>0, (λ_(p)−λ₀)>0, so that Δk<0. This can be the regime ofPA, and the nonlinearity helps to reduce the phase mismatch (i.e., thetwo parts in the second term in Equation (1) are of opposite sign).There can be gain for PA, and the gain can be tunable with the pumppower. For example, the power gain coefficient 2 g is plottedschematically in FIG. 5 for operation in the anomalous group velocityregime. The peak gain (g_(peak)=γP) can occur at Δk_(peak)=−2γP. Therange over which the gain exists can be given by 0>Δk>−4γP in someembodiments. Thus, the peak gain can be proportional to the pump power,and the Δk range can be determined by the pump power.

Consequently, from Equation (2) the bandwidth can be increased byincreasing the pump power, increasing the nonlinear coefficient n₂ ordecreasing the effective area A_(eff). In other embodiments, for a givenrequired frequency range over which gain is required, the pumprequirements can be reduced by increasing the effective nonlinearity(n₂/A_(eff)).

Several embodiments lead to broadband gain for cascaded Ramanamplification by arranging at least one intermediate Raman cascade orderat close proximity to the zero-dispersion wavelength λ₀ (e.g., within ±5nm of λ₀ for some embodiments; within ±2 nm for other embodiments).Either 4WM (if λ_(r)<λ₀) or PA (if λ_(r)>λ₀) can lead to spectralbroadening of that particular Raman order. In subsequent Raman ordersthe bandwidth can grow even further. In other embodiments, the cascadeRaman wavelength λ_(r) lies to the long wavelength side of λ₀ (i.e., inthe anomalous dispersion regime), so that parametric amplification canoccur.

An embodiment of the broadband NLPA is illustrated in FIG. 6. Startingfrom the pump wavelength λ_(p), cascaded Raman amplification can be usedin the first few stages. The pump can be more than one Raman shift or13.2 THz away from the zero-dispersion wavelength. To keep higherefficiency in these initial steps, some embodiments can use a narrowband cavity design, such as designs based on gratings or wavelengthselective couplers.

Some embodiments broaden the gain bandwidth by positioning one of theintermediate Raman cascade orders at a close proximity to thezero-dispersion wavelength λ₀. By operating close to λ₀, it can almostautomatically phase-match either 4WM or PA. In the subsequent cascadedRaman orders, the gain bandwidth may continue to broaden. This occursbecause the effective gain bandwidth of Raman is the convolution of thebandwidth of the pump (in this case, the previous Raman cascade order)with the Raman gain curve. In some embodiments with Raman amplification,the gain spectrum follows the pump spectrum. As the pump wavelengthchanges, the Raman gain can change as well, separated by the distance ofoptical phonon energy which in silica fibers is an approximately 13.2THz down-shift in frequency.

If the fiber is conventional so-called standard fiber, thenzero-dispersion wavelength λ₀ can be about 1310 nm. Fordispersion-shifted fiber, the zero-dispersion wavelength λ₀ can shift tolonger wavelengths by adding waveguide dispersion. In other embodiments,a dispersion-flattened fiber can be used for low dispersion values overone or more of the Raman cascade orders. In some embodiments withdispersion-flattened fiber, the dispersion slope can be small, so thegain bandwidth can be even larger (c.f. Equations (1) and (3)).

The Raman gain spectrum can follow the pump spectrum, such as when thereis nothing in the Raman cavity to restrict the bandwidth of thesubsequent orders. For these higher cascade order Raman laser schemes,some embodiments use gratings or wavelength selective couplers. Otherembodiments with the broadband cavity design of the Sagnac Ramanamplifier and laser can have increased bandwidth with a tailored pumpspectrum. A single-pass fiber design can constitute the broadestbandwidth design. A broadband cavity such as the Sagnac Raman cavity canhave the feedback used to lower the threshold and the required pumppower. Broadening the bandwidth can lead to a drop in efficiency, so thepump powers can be higher for the broadband cavity designs.

Cascaded Raman amplification can reach the 1430-1530 nm range of thelow-loss window. Pumping can occur with a commercially availablecladding-pumped fiber laser, which operates around 1060 to 1140 nm. Thevarious Raman orders, each separated by 13.2 Thz from the previousorder, are set forth in Table 1.

Wavelength (nm) Δλ 1060.00 51.86 1111.86 57.19 1169.05 63.39 1232.4470.66 1303.11 79.26 1382.37 89.53 1471.90 101.93 1573.82 117.09 1070.0052.86 1122.86 58.36 1181.22 64.76 1245.98 72.27 1318.25 81.17 1399.4291.82 1491.25 104.72 1595.97 120.54 1080.00 53.88 1133.88 59.54 1193.4266.14 1259.56 73.90 1333.47 83.11 1416.58 94.16 1510.74 107.57 1618.32124.07 1090.00 54.91 1144.91 60.74 1205.65 67.54 1273.19 75.56 1348.7485.09 1433.83 96.55 1530.38 110.49 1640.87 127.69 1100.00 55.95 1155.9561.94 1217.89 68.96 1286.85 77.24 1364.09 87.10 1451.19 98.98 1550.17113.47 1663.64 131.40 1110.00 57.00 1167.00 63.17 1230.16 70.40 1300.5678.94 1379.50 89.14 1468.64 101.46 1570.10 116.52 1686.62 135.20 1117.0057.74 1174.74 64.03 1238.77 71.41 1310.18 80.15 1390.33 90.59 1480.92103.22 1584.15 118.69 1702.84 137.92 1120.00 58.05 1178.05 64.40 1242.4671.85 1314.31 80.67 1394.98 91.22 1486.20 103.99 1590.19 119.63 1709.82139.10 1130.00 59.12 1189.12 65.65 1254.77 73.32 1328.10 82.43 1410.5393.33 1503.86 106.56 1610.42 122.81 1733.24 143.09 1140.00 60.20 1200.2066.92 1267.12 74.82 1341.93 84.21 1426.14 95.48 1521.62 109.18 1630.81126.07 1756.87 147.19

To obtain gain between 1430 nm and 1520 nm, the pump can be operatedbetween 1090 nm and 1140 nm, and five cascaded Raman orders can be usedto reach the desired wavelength. To make use of the broadening from PAor 4WM, a pumping scheme can be selected in the middle of this range,i.e., starting with a pump wavelength of 1117 nm. Then, the variousRaman orders land at approximately 1175 nm, 1240 nm, 1310 nm, 1390 nmand finally 1480 nm. In particular, the third Raman frequency (1310 nm)passes through the zero-dispersion point of a standard fiber, and thenext order (1390 nm) can be close if the fiber is dispersion shifted. Abroadband gain can be expected for wavelengths in the 1430-1530 nm rangecentered around 1480 nm by using a fiber with a standard dispersion anda pump wavelength of 1117 nm, 1175 nm or 1240 nm.

Broadening can be expected from PA. A standard fiber can be used and thepump wavelength can start at 1117 nm. The calculations use Equations(1-4) with the following typical parameters for high-Raman cross-sectionfiber in some embodiments: λ₀=1310 nm, γ=9.9 W⁻¹ km⁻¹, and a dispersionslope of 0.05 ps/nm−km. In FIG. 7, the gain coefficient for PA isplotted versus wavelength at a pump power of 1W and wavelengthseparations (λ_(r)−λ₀) of 0.5, 1, 2 and 5 nm. For a wavelengthseparation of 2 nm, the PA peak gain occurs at ±10 nm, so the spectralbroadening is over 20 nm. The closer the pump wavelength approaches thezero-dispersion wavelength, the wider the gain bandwidth can be. Inaddition, FIG. 8 plots the gain versus wavelength for a separation of(λ_(r)−λ₀)=1 nm and pump powers of 0.7, 1, 2, and 3W. The peak gain canincrease directly proportionally to the pump power, while the bandwidthcan increase as the square root of pump power.

FIG. 9 shows a first embodiment that uses an open-loop design to producean amplified broadband signal for a range of wavelengths between 1430 nmand 1530 nm. The open-loop design is a nonlinear polarization amplifier,and may have a high pump power requirement. In the NLPA amplifier 20 asillustrated in FIG. 9, an optical signal having a wavelength between1430 nm and 1530 nm is input from an input port 25 to an optical fiber30. The optical fiber 30 is pumped by a pumping light generated by apumping laser 35 operated at a wavelength of about 1240 nm. The opticalsignal is amplified and spectrally broadened in the fiber by nonlinearpolarization, and output through an output port 40. The configuration isso arranged that the optical signal can have a wavelength greater thanthe zero-dispersion wavelength of the fiber, which in turn is greaterthan the pumping wavelength of 1240 nm.

In this open-loop configuration, the fiber can have a cut-off wavelengthbelow 1240 nm to be single-mode (spatial) over all wavelengths of theRaman cascade. Three choices of the fiber embodiments can be used insome embodiments. First, a standard dispersion fiber with azero-dispersion wavelength at about 1310 nm. Second, two fibers splicedtogether with one fiber having a zero-dispersion wavelength at about1310 nm (first cascade) and the other at 1390 nm (second cascade).Third, a dispersion-flattened fiber with low-dispersion at least between1310 nm and 1390 nm. The reduced dispersion slope of such adispersion-flattened fiber increases significantly the bandwidth for PAor 4WM.

Exemplary 1240 nm pump lasers include: (a) an 1117 nm cladding-pumpedfiber laser followed by a coupler-based or grating-based Ramanoscillator cavity (with gratings for 1117 nm, 1175 nm and 1240 nm); (b)an optically-pumped semiconductor laser; or (c) a chromium-dopedforsterite laser. At one end of the fiber, a 1240 nm retro-reflector 45can be placed to increase pumping conversion efficiency. Theretro-reflector can be a dichroic mirror or a 1240 nm grating. The inputand output ports can be WDM couplers, and isolators can be used at theinput and output ports to prevent lasing due to spurious feedback. Acounter-propagating geometry can average out noise fluctuations in thisopen-loop configuration. A co-propagating geometry can be used.

To reduce the pump power requirements, a broadband cavity such as theSagnac Raman cavity can be used in some embodiments. FIG. 10 illustratesan embodiment of the NLPA that uses a Sagnac Raman cavity design with a1240 nm pump. Referring to FIG. 10, the Sagnac Raman cavity of the NLPA60 can be formed by a broadband mirror 70 and a loop mirror comprising aRaman gain fiber 65 and an optical coupler 90 connected thereto. Anoptical signal can have a wavelength between 1430 nm to 1530 nm inputthrough an input port 75 to the Raman gain fiber 65. A pumping laser 80can operate at a wavelength 1240 nm and generate a pumping light thatpumps the fiber 65 through a coupler 85. The optical signal can beamplified and spectrally broadened in the fiber by nonlinearpolarization, and output through an output port 95. The configurationcan be arranged so that the optical signal has a wavelength greater thanthe zero-dispersion wavelength of the fiber, which in turn can begreater than the pumping wavelength of 1240 nm.

The Raman gain fiber can have the same characteristics as describedabove for the open-loop design. Similarly, the pumping lasers used inthe first embodiment can be used in this second embodiment. Thebroadband NLPA may further include a polarization controller 100 in theSagnac Raman cavity for controlling polarization state. In otherembodiments, if the fiber is polarization maintained, the polarizationcontroller can be unnecessary. The optical coupler 90 is nominally 50:50at least for the optical signal having a wavelength between about 1240nm and 1430 nm. The coupler 85 can be a WDM coupler that transmits atleast at a wavelength between about 1300 nm and 1430 nm. The input portand output port each comprises a WDM coupler which can transmit at leastat a wavelength between about 1240 nm and 1425 nm. One embodiment of theSagnac Raman cavity has a passive noise dampening property that leads toquieter cascading of various Raman orders.

In various embodiments, a Sagnac Raman cavity can be used for all fiveRaman cascade orders between 1117 nm and the low-loss window. FIG. 11illustrates a third embodiment of a five-order Sagnac Raman amplifierfor NLPA operation. A cladding-pumped fiber laser operating around 1117nm can be used as a pumping laser 120. Different fiber combinationsembodiment can be used. The fibers can have a cut-off wavelength below1117 nm to accommodate single-mode operation for the pump. An opticalcoupler 130 can be nominally 50:50 at least for the optical signalhaving the wavelength between about 1117 nm and 1430 nm. A coupler 125can be a WDM coupler that transmits at least at wavelengths betweenabout 1165 nm and 1430 nm. Moreover, the input and output ports eachcomprises a WDM coupler which can transmit at least at wavelengthsbetween about 1117 nm and 1425 nm. Although the wavelength range of thevarious components increases, this configuration can lead to an evenbroader gain band since the pump bandwidth is allowed to increase evenduring the first two cascades between 1117 nm and 1240 nm for someembodiments. Also, the noise dampening property of the Sagnac cavity canbe used over all five Raman orders for some embodiments.

Some embodiments include an NLPA. An optical signal having a wavelengthλ is input through an input port into a distributed gain medium havingzero-dispersion at a wavelength λ₀, such as an optical fiber, which canbe pumped by a pumping light from a pump source operated at a wavelengthλ_(p), wherein λ≧λ₀≧λ_(p). The pumping light can cascade through thedistributed gain medium a plurality of Raman orders including anintermediate order having a wavelength λ_(r) at a close proximity to thezero-dispersion wavelength λ₀ to phase match four-wave mixing (ifλ_(r)<λ₀) or parametric amplification (if λ_(r)>λ₀). The amplified andspectrally broadened optical signal is output through an output port.

The above embodiments demonstrate that a single NLPA can accommodate thefull bandwidth of the low-loss window. Moreover, the full bandwidth ofthe low-loss window may be reached by using a parallel opticalamplification apparatus having a combination of two or more Ramanamplifiers and rare earth doped amplifiers. In some embodiments, theNLPAs and EDFAs are used.

FIG. 12 shows a first embodiment of the parallel optical amplificationapparatus using a combination of two NLPAs for a range of wavelengthsbetween 1430 nm and 1530 nm. Referring to FIG. 12, a divider 170 dividesan optical signal having a wavelength between 1430 nm to 1530 nm at apredetermined wavelength, such as 1480 nm, into a first beam having awavelength less than the predetermined wavelength and a second beamhaving a wavelength greater than the predetermined wavelength in someembodiments. The first beam is input into a first NLPA 180 foramplification and spectral broadening therein. The second beam is inputinto a second NLPA 190 for amplification and spectral broadeningtherein. Outputs from the first and second NLPAs can be combined by acombiner 200 to produce an amplified and spectrally broadened opticalsignal. The input port 170 and output port 200 can be preferably WDMcouplers in some embodiments.

In other embodiments the first NLPA 180 can be optimized for 1430-1480nm and centered at 1455 nm, while the second NLPA can be optimized for1480-1530 nm and centered at 1505 nm. From Table 1, these two windowscan be achieved in a five-order cascade by starting with a pumpwavelength of about 1100 nm for the short-wavelength side and a pumpwavelength of about 1130 nm for the long-wavelength side. For theshort-wavelength side, the fiber can have a zero-dispersion around 1365nm, while for the long-wavelength side, the fiber zero-dispersion can bearound 1328 nm or 1410 nm.

The narrower-bandwidth for each NLPA can lead to an increased efficiencyfor each amplifier in some embodiments. Furthermore, the components maybe more easily manufactured, since the wavelength window is not aslarge. The multiple amplifiers in some embodiments may allow for gradualupgrades of systems, adding bandwidth to the EDFA window as needed.

A spectrum of 1430-1620 nm in the low-loss window can be amplified andspectrally broadened by using a parallel optical amplification apparatuscomprising Raman amplifiers and rare earth doped amplifiers. FIG. 13describes a second embodiment of the parallel optical amplificationapparatus. The amplification apparatus comprises a broadband NLPA 240and a EDFA 250. A divider 230 of the apparatus divides an optical signalhaving a wavelength between 1430 nm and 1620 nm at a predeterminedwavelength, preferably at 1525 nm, into a first beam having a wavelengthless than the predetermined wavelength and a second beam having awavelength greater than the predetermined wavelength in someembodiments. The broadband NLPA 240 receives the first beam and producesan amplified broadband first beam. The EDFA 250 receives the second beamand produces an amplified broadband second beam. A combiner 260 combinesthe amplified and spectrally broadened first and second beams to producean amplified broadband optical signal. Other embodiments can have WDMcouplers for the divider 230 and the combiner 260.

To use some embodiments with multi-wavelength WDM channels, at theoutput of the amplifier, gain can be equalized. This wavelengthdependency or nonuniformity of the gain band can have little impact onsingle-channel transmission. However, it can render the amplifierunsuitable for multichannel operation through a cascade of amplifiers.As channels at different wavelengths propagate through a chain ofamplifiers, they can accumulate increasing discrepancies between them interms of gain and signal-to-noise ratio. Using gain-flattening elementscan significantly increase the usable bandwidth of a long chain ofamplifiers. For example, the NLPA can be followed by a gain flatteningelement to provide gain equalization for different channels in someembodiments. Alternately, the gain flattening element could beintroduced directly into the Sagnac interferometer loop in otherembodiments, such as in FIG. 10 or 11.

Due to the high pump power requirements of Raman amplifiers, someembodiments include higher efficiency Raman amplifiers, where theefficiency can be defined as the ratio of signal output to pump input.In one embodiment, the efficiency can be improved to the point thatlaser diodes (LD's) can be used to directly pump the Raman amplifier. Asan exemplary benchmark, for a dual stage amplifier made fromdispersion-shifted fiber (DSF) with a gain of >15 dB and an electricalnoise figure of <6 dB, a pump power of about 1 W can be required fromthe Raman oscillator or pump laser. This power level can require thecombined powers from about eight LD's in one embodiment. If the pumprequirements could be dropped by a factor of four or so, the pump powerscould be achieved with the combination of two LD's that are polarizationmultiplexed in another embodiment. In one embodiment, four LD's could beused to provide more than 0.5W of power, and the remaining improvementfactor could be used to reduce the gain fiber lengths.

One embodiment improves the efficiency of Raman amplifiers by increasingthe effective nonlinearity of the fiber used as the gain medium. Theeffective nonlinear coefficient for the fiber can be defined as$\gamma = {\frac{2\pi}{\lambda}\frac{n_{2}}{A_{eff}}}$

where n₂ is the nonlinear index of refraction and A_(eff) is theeffective area of the fiber. The Raman gain coefficient can be directlyproportional to γ. The Raman coefficient is the imaginary part of thenonlinear susceptibility while the index is proportional to the realpart of the susceptibility, and the nonlinear index and Raman gain willbe related by the so-called Kramers-Kronig relations. For a dispersionshifted fiber at 1550 nm wavelength with an n₂=2.6×10⁻¹⁶ cm²/W and anA_(eff)=50 μm², the nonlinear coefficient can be about γ=2 W⁻¹ km⁻¹. Ifthis value is raised to over 3 W⁻¹ km⁻¹, then the pump power or fiberlengths can be reduced in proportion to the increase in nonlinearcoefficient.

Beyond the constraint on the Raman gain coefficient, the dispersion inthe amplifier can be restricted. To maintain a relatively low level ofdispersion in the vicinity of the signal wavelengths, the zerodispersion wavelength λ_(o) can be in close proximity to the operatingwavelength. For single-channel, high-bit-rate systems, one embodimentminimizes the dispersion by placing the signal wavelength within 10 nmof the λ_(o). For some embodiments of multi-wavelength WDM systems,where the channels can interact through four-wave mixing in the vicinityof λ_(o), a dispersion-managed fiber can be used. A dispersion-managedfiber can have a locally high dispersion but a path-averaged value fordispersion close to zero by combining lengths of plus and minus valuesfor the dispersion around the operating band. For the operatingwavelength band, some segments of fiber can have λ_(o) at shorterwavelengths and some segments of fiber can have λ_(o) at longerwavelengths.

By proper design of the fiber, higher nonlinearity and lower dispersioncan be achieved. For example, for operation in the S-band around 1520nm, high nonlinearity fibers have been produced. The fiber core can havea modified parabolic refractive index profile with a Δ_(peak)=2%. Threeexemplary fibers have zero dispersion wavelengths of 1524 nm, 1533 nmand 1536 nm. Such fibers can have a dispersion slope of 0.043 ps/nm²−km,and the loss at 1550 nm can be approximately 0.6 dB/km. The nonlinearcoefficient can be γ=9 W⁻¹ km⁻¹, or a factor of 4.5× higher than in DSF.The enhancement can be attributed to two factors: a smaller effectivearea and a higher germanium content. The effective area can be reducedto about A_(eff)=16.5 μm², or about a factor of 3.3 less than in DSF.Also, the nonlinear index of refraction is about 1.35× larger than inDSF due to the extra germanium used to increase Δ_(peak) from 1% in DSFto 2% for the high nonlinearity fiber. In addition the mode fielddiameter at 1550 nm can be measured to be 4.67 μm.

For the gain fiber used in the Raman amplifier, a figure-of-merit forthe fiber can be defined in some embodiments. A figure-of-merit that canbe measured and indicate amplifier performance is the ratio of the Ramangain coefficient to the loss at the signal wavelength. The higher thisfigure-of-merit, the better the performance of the amplifier. Thisfigure-of-merit for different fibers in some embodiments is provided inTable 1. In one embodiment the lowest figure-of-merit is found forstandard (non-dispersion-shifted) SMF-28 fiber. This fiber can have alow germanium content and a relatively large A_(eff)=86 μm². Thefigures-of-merit for the high-nonlinearity (Hi-NL) fiber can exceed theother fibers, with a value about two-fold larger than Lucent True-wavefiber in one example. Although the DCF's can have a relatively largefigure-of-merit for Raman amplification, they can have very largedispersion coefficients for S-band signals.

TABLE 1 Comparison of Raman gain figure-of-merit for different fibersmeasured. Gain [dB/W-km] Loss [dB/km] Fiber Type @ 1500 nm @1500 nmFigure-of-Merit Corning SMF-28 2.2 0.19 11.6 Lucent True-Wave 3.3 0.2115.7 Corning SMF-DS 4.0 0.2 20.0 Corning DCF 11.75 0.445 26.4 Lucent DCF13.72 0.5 27.6 Hi-NL 18.0 0.6 30.0

One embodiment with Hi-NL fiber has significant improvements in terms offiber length and pump power used in a Raman amplifier. One embodimenthas an amplifier made out of Lucent True-Wave fiber. The specificationsfor the unit can be: low dispersion around 1520 nm, 15 dB of peak gain,electrical and optical NF under 6 dB, and multi-path interference (MPI)under 50 dB. A two-stage design for the Raman amplifier can be used, asillustrated in FIG. 14. In particular, 6 km of True-Wave fiber can beused in the first stage and 10-12 km of fiber can be used in the secondstage. The measured performance of the amplifier can be: peak gain of15.2 dB at 1516 nm, 3 dB bandwidth of 26 nm (between 1503-1529 nm), andelectrical and optical noise figure under 6 dB. For example, the gainversus wavelength and noise figure versus wavelength for the unit isillustrated in FIGS. 15 and 16. This performance can have a pump powerof about 1.0 W at 1421 nm.

In one embodiment, the True-Wave fiber in this design is replaced withHi-NL fiber. Reductions in fiber lengths and pump power requirements canbe achieved. The Hi-NL fiber can meet the dispersion requirement in someembodiments. The DCF fibers can lead to the introduction of largeamounts of dispersion. Referring to the Table 1 comparison, the fiberlengths can be chosen to keep roughly the same amount of net loss. Inone embodiment, fiber lengths can be roughly 2 km for the first stageand 3.3-4 km for the second stage. Pump power requirements can belowered by the ratio of figures-of-merit, or roughly to 0.5 W. invarious embodiments, this power range can be provided by the Ramanoscillator, or by polarization and wavelength multiplexing 3-4 LD'stogether. Hi-NL fiber can reduce the size of the amplifier as well aspermit LD pumping in some embodiments.

The fiber can have single-mode operation for the pump as well as thesignal wavelengths in some embodiments. Cut-off wavelength λ_(c) of thefiber can be shorter than any of the pump wavelengths in someembodiments. The pump can be multi-mode, and noise can be introducedfrom the beating between modes in other embodiments.

Various embodiments have reduction of the Raman amplifier size and pumprequirements while maintaining low net dispersion at the operatingwavelengths, and include one or more of:

(A) A Raman amplifier using a gain fiber characterized in that

nonlinear coefficient γ>3 W⁻¹ km⁻¹

zero dispersion wavelength in the range of 1300<λ_(o)<1800 nm, dependingmore precisely on the specifications

Loss over the operating wavelength of <2 dB/km, with a preference forloss<1 dB/km

(B) A Raman amplifier using a dispersion managed gain fibercharacterized in that

nonlinear coefficient γ>3 W⁻¹ km⁻¹

dispersion management done using segments of fiber with zero dispersionwavelength in the range of 1300<λ_(o)<1800 nm, depending more preciselyon the specifications. Given an operating band, certain fiber segmentshave λ_(o) less than the operating band and other fiber segments haveλ_(o) greater than the operating band. The local dispersion can be kepthigh, while the path average dispersion can be close to zero in thesignal band.

Loss over the operating wavelength of certainly<2 dB/km, with apreference for loss<1 dB/km

(C) Fibers as in (A) or (B) with cut-off wavelength shorter than any ofthe pump wavelengths.

(D) A Raman amplifier as described in (A) that is pumped by LD's. Fortwo or more LD's, the power can be combined by using polarization andwavelength multiplexing using polarization beam combiners andwavelength-division-multiplexers.

(E) A Raman amplifier as in (B) that is pumped by LD's. For two or moreLD's, the power can be combined by using polarization and wavelengthmultiplexing using polarization beam combiners andwavelength-division-multiplexers.

(F) At least a two-stage Raman amplifier that uses the improvements in(A), (B), (C), (D) or (E).

(G) Other factors as above with different numerical ranges

Some embodiments include standard dispersion fiber, i.e., fibers withzero dispersion wavelength around 1310 nm. The zero dispersionwavelength can fall in the S− or S⁺-bands in some embodiments. Forexample, this is true for so-called non-zero-dispersion-shifted fiber(NZ-DSF). In these fibers, it can be difficult to run multi-wavelengthWDM channels due to cross-talk from four-wave mixing. Four-wave-mixingcan require phase matching, and the phase matching can be easier tosatisfy in the neighborhood of the zero dispersion wavelength. Oneembodiment is a broadband fiber transmission system with non-zerodispersion fiber that has zero dispersion wavelengths less than 1540 nmor greater than 1560 nm that uses optical amplifiers to compensate forloss.

WDM can maximize capacity in any given band in some embodiments. Hybridamplifiers can be useful in the vicinity of the zero dispersionwavelength in some embodiments. NZ-DSF fibers can have a zero dispersionwavelength either <1540 nm or >1560 nm in some embodiments. Foroperation near the zero dispersion wavelength, e.g., |λ−λ_(o)|<25 nm,the four-wave-mixing penalty can be avoided by using hybrid opticalamplifiers in one embodiment. Since the effective NF of hybridamplifiers can be lower than for discrete amplifiers, the power levelsfor the signals can be reduced to the point that four-wave-mixing can nolonger be a limitation, in another embodiment.

One embodiment of a broadband fiber transmission system comprises atransmission line and one or more semiconductor lasers. The transmissionline can have at least one zero dispersion wavelength λ_(o). Thetransmission line can transmit an optical signal of λ. The opticalsignal can have a wavelength λ in the range of 1430 nm and 1530 nm, orin the range of 1530 nm and 1630 nm. The signal wavelength at λ can besufficiently low in power to avoid at least one fiber non-linearityeffect. The at least one fiber non-linearity effect can comprisefour-wave mixing, and/or modulation instability. λ can be close to λ₀. λcan be within 20 nm, or within 30 nm λ₀. λ₀ can be less than 1540 nm,and/or greater than 1560 nm. The transmission line can include adistributed Raman amplifier. The distributed Raman amplifier can amplifythe optical signal through Raman gain. The distributed Raman amplifiercan have sufficient gain to compensate for losses in the transmissionline. The one or more semiconductor lasers can operate at wavelengthsλ_(p) for generating a pump light to pump the Raman amplifier.

One embodiment of a broadband fiber transmission system comprises atransmission line and one or more semiconductor lasers. The transmissionline can have at least one zero dispersion wavelength λ_(o). Thetransmission line can transmit an optical signal of λ. The opticalsignal can have a wavelength λ in the range of 1430 nm and 1530 nm, orin the range of 1530 nm and 1630 nm. The signal wavelength at λ can besufficiently low in power to avoid at least one fiber non-linearityeffect, such as four-wave mixing, and/or modulation instability. Thetransmission line can include a distributed Raman amplifier. Thetransmission line can include a discrete optical amplifier. Theamplifiers can have sufficient gain to compensate for losses in thetransmission line. The discrete optical amplifier can amplify theoptical signal of λ. The discrete optical amplifier can be a rare earthdoped amplifier, an erbium doped fiber amplifier, a Raman amplifier,and/or a thulium doped fiber amplifier. The one or more semiconductorlasers can operate at wavelengths λ_(p) for generating a pump light topump the amplifiers. λ can be close to λ_(o). λ can be within 30 nm, orwithin 20 nm, of λ_(o). λ_(o) can be less than 1540 nm, and/or greaterthan 1560 nm.

One embodiment of a method of broadband amplification comprisesproviding a broadband fiber transmission system with a transmission linehaving at least one zero dispersion wavelength λ_(o), the transmissionline including a distributed Raman amplifier that amplifies an opticalsignal through Raman gain; transmitting an optical signal of λ; pumpingthe Raman amplifier with pump light λ_(p), wherein λ is close to λ_(o)and λ_(o) is less than 1540 nm or greater than 1560 nm. λ can be within30 nm, or within 20 nm, of λ_(o). The optical signal can have awavelength λ in the range of 1430 nm and 1530 nm, or in the range of1530 nm and 1630 nm. The signal wavelength at λ can be sufficiently lowin power to avoid at least one fiber non-linearity effect, such asfour-wave mixing, and/or modulation instability. The distributed Ramanamplifier can have sufficient gain to compensate for losses in thetransmission line.

One embodiment of a method of broadband amplification comprisesproviding a broadband fiber transmission system with a transmission linehaving at least one zero dispersion wavelength λ_(o), the transmissionline including a distributed Raman amplifier and a discrete opticalamplifier that amplify an optical signal of λ; transmitting an opticalsignal of λ; pumping the Raman amplifier and discrete optical amplifierswith pump light λ_(p), wherein λ is close to λ_(o) and λ_(o) is lessthan 1540 nm or greater than 1560 nm. λ can be within 30 nm, or within20 nm, of λ_(o). The optical signal can have a wavelength λ in the rangeof 1430 nm and 1530 nm, or in the range of 1530 nm and 1630 nm. Thediscrete optical amplifier can be a rare earth doped amplifier, anerbium doped fiber amplifier, a Raman amplifier, and/or a thulium dopedfiber amplifier. The signal wavelength at λ can be sufficiently low inpower to avoid at least one fiber non-linearity effect, such asfour-wave mixing, and/or modulation instability. The amplifiers can havesufficient gain to compensate for losses in the transmission line.

Various other modifications can be readily apparent to those skilled inthe art without departing from the scope and spirit of the invention.Accordingly, it is not intended that the scope of the claims appendedhereto be limited to the description set forth herein, but rather thatthe claims be construed as encompassing all the features of thepatentable novelty that reside in the present invention, including allfeatures that would be treated as equivalents thereof by those skilledin the art to which this invention pertains.

What is claimed is:
 1. A broadband fiber transmission system,comprising: a transmission line having at least a first zero dispersionwavelength λ₀₁ and a second zero dispersion wavelength λ₀₂, thetransmission line operable to transmit an optical signal of λ, thetransmission line including a Raman gain medium that amplifies theoptical signal through Raman gain; and one or more semiconductor lasersoperated at wavelengths λ_(p) for generating a pump light to pump theRaman amplifier, wherein λ is within 30 nm of at least one of λ₀₁ andλ₀₂, and wherein λ₀₁ is separated from λ₀₂ by at least 50 nm.
 2. Thebroadband fiber transmission system of claim 1, wherein λ is within 20nm of at least one of λ₀₁ and λ₀₂.
 3. The broadband fiber transmissionsystem of claim 1, wherein the optical signal has a wavelength λ in therange of 1430 nm and 1530 nm.
 4. The broadband fiber transmission systemof claim 1, wherein the optical signal has a wavelength λ in the rangeof 1530 nm and 1630 nm.
 5. The broadband fiber transmission system ofclaim 1, wherein a signal wavelength at λ is sufficiently low in powerto avoid at least one fiber non-linearity effect.
 6. The broadband fibertransmission system of claim 1, wherein the Raman gain medium hassufficient gain to compensate for losses in the transmission line. 7.The broadband fiber transmission system of claim 5, wherein the at leastone fiber non-linearity effect comprises four-wave mixing.
 8. Thebroadband fiber transmission system of claim 5, wherein the one fibernon-linearity effect is modulation instability.
 9. A method of broadbandamplification, comprising: providing a broadband fiber transmissionsystem with a transmission line having at least a first zero dispersionwavelength λ₀₁ and a second zero dispersion wavelength λ₀₂, thetransmission line including a Raman gain medium tat amplifies an opticalsignal through Raman gain; transmitting an optical signal of λ; pumpingthe Raman amplifier with pump light λ_(p), wherein λ is within 30 nm ofat least one of λ₀₁ and λ₀₂, and wherein λ₀₁ is separated from λ₀₂ by atleast 50 nm.
 10. The method of claim 9, wherein λ is within 20 nm of atleast one of λ₀₁ and λ₀₂.
 11. The method of claim 9, wherein the opticalsignal has a wavelength λ in the range of 1430 nm and 1530 nm.
 12. Themethod of claim 9, wherein the optical signal has a wavelength λ in therange of 1530 nm and 1630 nm.
 13. The method of claim 9, wherein asignal wavelength at λ is sufficiently low in power to avoid at leastone fiber non-linearity effect.
 14. A The method of claim 9, wherein theRaman gain medium has sufficient gain to compensate for losses in thetransmission line.
 15. The method of claim 13, wherein the at least onefiber non-linearity effect comprises four-wave mixing.
 16. The method ofclaim 15, wherein the at least one fiber non-linearity effect comprisesmodulation instability.
 17. A broadband fiber transmission system ofclaim 1, wherein the transmission line comprises at least a firstoptical fiber and a second optical fiber coupled to the first opticalfiber.
 18. A broadband fiber transmission system of claim 1, wherein thetransmission line comprises at least a first portion having a positivesign of dispersion and a second portion having a negative sign ofdispersion.
 19. The broadband fiber transmission system of claim 1,wherein the Raman gain medium comprises a distributed Raman gain medium.20. The broadband fiber transmission system of claim 1, wherein theRaman gain medium comprises a discrete Raman gain medium.
 21. Thebroadband fiber transmission system of claim 1, wherein the pump lightgenerated by the at least one semiconductor laser propagates the Ramangain medium in a direction that is direction that the optical signalpropagates the Raman gain medium.
 22. The broadband fiber transmissionsystem of claim 1, wherein the first zero dispersion wavelength λ₀₁ isseparated from the second zero dispersion wavelength λ₀₂ by at least 80nm.
 23. The broadband fiber transmission system of claim 1, wherein λcomprises one of a plurality of signal wavelengths forming the opticalsignal.
 24. The broadband fiber transmission system of claim 1, furthercomprising: an input port operable to input the pump light generated bythe one or more semiconductor lasers to the Raman gain medium; and anoutput port operable to remove at least a portion of the pump light fromthe Raman gain medium.
 25. The method of claim 9, wherein thetransmission line comprises at least a first optical fiber and a secondoptical fiber coupled to the first optical fiber.
 26. The method ofclaim 9, wherein the transmission line comprises at least a firstportion having a positive sigh of dispersion and a second portion havinga negative sign of dispersion.
 27. The method of claim 9, wherein thepump light propagates the Raman gain medium in a direction that issubstantially opposite the direction that the optical signal propagatesthe Raman gain medium.
 28. The method of claim 9, wherein the first zerodispersion wavelength λ₀₁ is separated from the second zero dispersionwavelength λ₀₂ by at least 80 nm.
 29. The method of claim 9, wherein λcomprises one of a plurality of signal wavelengths forming the opticalsignal.
 30. The method of claim 9, further comprising: inputting thepump light to the Raman gain medium; and removing at least a portion ofthe pump light from the Raman gain medium.